TR

Ecology: Communitiy

  • Community: an assemblage of populations of various species living close enough for potential interaction

Ecological Niches and Natural Selection

  • Ecological niche: the total of a species’ use of biotic and abiotic resources
    • an ecological niche can also be thought of as an organism’s ecological role
  • Ecologically similar species can coexist in a community if there are one or more significant differences in their niches
  • Resource partitioning: differentiation of ecological niches, enabling similar species to coexist in a community

Niches

  • Fundamental niche: the niche potentially occupied by that species
  • Realized niche: the niche actually occupied by that species
  • As a result of competition, a species’ fundamental niche may differ from its realized niche

Types of Community Interactions

  • Interspecific interactions: relationships between species in a community
    • Competition, predation, herbivory, symbiosis (parasitism, mutualism, and commensalism), and facilitation
  • Interspecific interactions can affect the survival and reproduction of each species,
    • positive (+), negative (–), or no effect (0)

Competition

  • Intraspecific competition: competition between individuals of the same species
  • Interspecific competition: competition between individuals of different species
  • Exploitation competition: organisms compete indirectly through the consumption of a limited resource
  • Interference competition: individuals interact directly with one another by physical force or intimidation
  • Strong competition can lead to competitive exclusion, local elimination of a competing species
    • The competitive exclusion principle states that two species competing for the same limiting resources cannot coexist in the same place

Predation

  • Predation (+/– interaction): interaction where one species, the predator, kills and eats the other, the prey
    • Some feeding adaptations of predators are claws, teeth, fangs, stingers, and poison
    • Prey display various defensive adaptations
    • Behavioral defenses include hiding, fleeing, forming herds or schools, self-defense, and alarm calls
    • Animals also have morphological and physiological defense adaptations
    • Cryptic coloration, or camouflage, makes prey difficult to spot

Antipredator Strategies

  • Chemical defense 
    • ex: Bombardier beetle ejects hot spray
  • Displays of intimidation: deceive predator about ease of eating prey
    • ex: Porcupine fish inflates itself
  • Importance of predation depends on whether it’s a donor-controlled system or a predator-controlled system
    • Donor-controlled system: prey supply is determined by factors other than predation, such as food supply
    • Removal of predators has no effect on prey density
    • Predator-controlled system: action of predator feeding reduces supply of prey
    • Removal of predator results in increase in prey

Plants vs Herbivores

  • Herbivory can be lethal to small species
  • Usually not lethal to larger species
  • Generalist herbivores: can feed on many plant species
  • Specialist herbivores: restricted to one or two host plants
  • Two proposals explain why every plant isn’t consumed
    • Predators and parasites keep herbivore numbers low
    • Plant defenses make a difference

Plants Defenses

  • Array of unusual and powerful chemicals

    • Secondary metabolites: not part of primary \n energy-generating metabolic pathway
    • Alkaloids(nicotine in tobacco, morphine in poppies, cocaine in coca, and caffeine in coffee)
    • Phenolics (lignin in wood and tannin in leaves)
    • Terpenoids (in peppermint)

  • Mechanical defenses like thorns and spines

  • Herbivores can overcome plant defenses

    • Detoxify using two pathways
    • Oxidation: catalysis of secondary metabolite to corresponding alcohol by mixed-function oxidases (MFOs)
    • Conjugation: unites results of oxidation with another molecule to create inactive and readily excreted product

  • Parasitism (+/– interaction): one organism, the parasite, derives nourishment from another organism, its host, which is harmed in the process (does not usually kill it outright).

    • Endoparasites: parasites that live within the body of their host
    • Ectoparasites: parasites that live on the external surface of a host

Mutualism

  • Mutualism: close associations between species in which both species benefit
  • Trophic mutualism: both species utilize a common resource
    • Example: Leaf-cutting ants and fungus
    • Ants chew up leaves to feed to fungus they care for
    • Fungus produces gongylidia as ant food
    • Ants circumvent chemical defenses of leaves
  • Defensive mutualism: animal defends a plant or herbivore
    • ex: Ants protect aphid, aphid secretes honeydew
    • Facultative mutualism: the species can’t live apart
    • ex: Ants nesting in acacia trees defend the tree and trim away competing plants
    • Obligatory mutualism: neither species can live alone
  • Dispersive mutualism: one species receives food in exchange for helping the flower spread their pollen
    • Plant would like pollinator with fidelity to one species that moves quickly among individuals
    • Animal wants to be a generalist to get the most food in a small area, reducing energy expenditure
    • Mutualisms are beneficial – but optimal needs of each party can be different

Commensalism

  • Commensalism: one member derives a benefit while the other is not benefited or harmed
  • ex: Epiphytes growing in trees do not harm the trees
  • ex: Cattle egrets benefit from cattle stirring up insects
  • Phoresy: one organism uses another for transportation
    • Flower-inhabiting mites use hummingbird nostrils
  • Cheating
    • Grass-pink orchid produces no nectar, but it mimics the nectar-producing rose pogonia and is therefore still visited by bees
    • Plants cheat seed-dispersal agent out of meal with barbs or hooks on seeds

Facilitation

  • Facilitation (+/+ or 0/+): an interaction where one species can have positive effects on another species without direct and intimate contact

    • For example, the black rush makes the soil more hospitable for other plant species

  • Key distinction between three models is in the manner succession proceeds

    • Facilitation: species replacement is assisted by previous colonists
    • Inhibition: species replacement is prevented by previous colonists
    • Tolerance: species replacement is unaffected by previous colonists

  • Other factors may also influence succession

Species Diversity

  • Species diversity: the variety of organisms that make up the community
  • It has two components: species richness and relative abundance
    • Species richness: the total number of different species in the community
    • Relative abundance: the proportion each species represents of the total individuals in the community
  • Communities with higher diversity are
    • More productive and more stable in their productivity
    • Better able to withstand and recover from environmental stresses
    • More resistant to invasive species, organisms that become established outside their native range

Trophic Structure

  • Trophic structure: the feeding relationships between organisms in a community
  • It is a key factor in community dynamics
  • Food chains link trophic levels from producers to top carnivores

Species with a Large Impact

  • Dominant species: the species that are most abundant or have the highest biomass

    • Dominant species exert powerful control over the occurrence and distribution of other species

    • Sugar maples have a major impact on shading and soil nutrient availability in eastern North America 🡪affects the distribution of other plant species

    • Dominant species are most competitive in exploiting resources vs they are most successful at avoiding predators

      • Invasive species, typically introduced to a new environment by humans, often lack predators or disease

  • Keystone species: exert strong control on a community by their ecological roles, or niches

    • They are not necessarily abundant in a community

Ecological Succession

  • Ecological succession: the sequence of community and ecosystem changes after a disturbance
  • Primary succession: when succession begins; occurs where no soil exists
  • Secondary succession: begins in an area where soil remains
    • Early-arriving species and later-arriving species may be linked in one of three processes
    • Early arrivals 
      • May facilitate appearance of later species by making the environment favorable
      • May inhibit establishment of later species
      • May tolerate later species but have no impact on their establishment